Background: The 2023-2024 influenza season included sustained elevated activity from December 2023-February 2024 and continued activity through May 2024. Influenza A(H1N1), A(H3N2), and B viruses circulated during the season.
Methods: During September 1, 2023-May 31, 2024, a multistate sentinel surveillance network of 24 medical centers in 20 U.S. states enrolled adults aged ≥18 years hospitalized with acute respiratory illness (ARI). Consistent with a test-negative design, cases tested positive for influenza viruses by molecular or antigen test, and controls tested negative for influenza viruses and SARS-CoV-2. Vaccine effectiveness (VE) against influenza-associated hospitalization was calculated as (1 - adjusted odds ratio for vaccination) × 100%.
Results: Among 7690 patients, including 1170 influenza cases (33% vaccinated) and 6520 controls, VE was 40% (95% CI: 31%-48%) with varying estimates by age (18-49 years: 53% [34%-67%]; 50-64 years: 47% [31%-60%]; ≥65 years: 31% [16%-43%]). Protection was similar among immunocompetent patients (40% [30%-49%]) and immunocompromised patients (32% [7-50%]). VE was statistically significant against influenza B (67% [35%-84%]) and A(H1N1) (36% [21%-48%]) and crossed the null against A(H3N2) (19% [-8%-39%]). VE was higher for patients 14-60 days from vaccination (54% [40%-65%]) than >120 days (18% [-1%-33%]).
Conclusions: During 2023-2024, influenza vaccination reduced the risk of influenza A(H1N1)- and influenza B-associated hospitalizations among adults; effectiveness was lower in patients vaccinated >120 days prior to illness onset compared with those vaccinated 14-60 days prior.
Keywords: hospitalization; seasonal influenza; severe influenza; vaccine effectiveness; waning.
Published by Oxford University Press on behalf of Infectious Diseases Society of America 2025.